Fractures are a common form of musculoskeletal injury. While the inherent repair capability of bone tissue results in recovery of non-critical injuries, larger segmental bone defects are more challenging. Left untreated, these segmental defects can lead to non-unions. A number of stimuli are known to enhance bone regeneration and include bone morphogenetic protein - 2 (BMP-2) and low intensity pulsed ultrasound (LIPUS). It is presently unknown how the regenerative process in a large segmental defect will respond to a combination of these two distinct modalities. The hypothesis to be addressed is that simultaneous treatment of segmental bone defects with low intensity pulsed ultrasound and bone morphogenetic protein-2 will act synergistically or additively to enhance bone regeneration. The hypothesis is based on recent observations by our group that LIPUS enhanced the amount of new bone formation initiated by recombinant human (rh)BMP-2 in an ectopic bone formation model in the rat. Our objective is to determine if addition of LIPUS to rhBMP-2 treatment of a segmental bone defect in a well-characterized rat model accelerates healing so that better treatment protocols can be developed for improved bone healing. In Aim 1, we will study the response of segmental bone defects to varying doses of rhBMP-2 (1 - 205g) loaded on to absorbable collagen sponges in order to determine the optimum dose of rhBMP-2. Healing will be monitored by in vivo radiographs. The primary endpoints will be bone volume (micro computed tomography and histology) and mechanical strength (torsional testing). In Aim 2, we will test the ability of LIPUS to enhance the repair process in the presence of optimum and sub-optimum doses of rhBMP-2 based on information obtained in Aim 1. The primary endpoints will be the same as for Aim 1. In Aim 3, we will identify the most responsive phase(s) of rhBMP-2-induced bone formation to LIPUS treatment in the best format of combined treatment. The primary endpoints will be the same as for Aim 1. Information from this study can be readily translated to clinical situation as both rhBMP-2 and LIPUS are already approved for clinical use. The proposed research also could benefit patients undergoing surgical procedures such as joint replacement, distraction osteogenesis and dental implant placement in which bone regeneration is integral to successful outcomes. The approach proposed here might also serve as a model for the treatment of other musculoskeletal tissues (e.g., muscles, nerves, ligaments and tendons). PUBLIC HEALTH RELEVANCE: The relevance of the project to Public Health is that aiding the inherent ability of bone to repair and regenerate will reduce the risk of complications associated with subnormal healing and possible non-union. Fractures are a common form of musculoskeletal injury;there are roughly 7 million fractures per year in the United States alone. This research will focus on enhancing bone regeneration by applying a combination of two already approved FDA treatments and could be rapidly translated into clinical use.